Process for decarburizing austenitic manganese cast iron



Patented Feb. 10, 1948 PROCESS FOR DECARBURIZIN G AUSTEN- ITIG MANGANESECAST IRON Alfred Gordon Evans Robiette and Peter Francis Hancock,Erdington, Birmingham, signors to Birlec Limited, Birmingham,

land

No Drawing. Application In Great England, as- Eng- June 30, 1943, SerialBritain February 27,

4 Claims. (01. 148-16) The present inventionrelatesto improved ironalloys, and to a process for treating castings of austenitic white castiron alloys, in which the principal alloying agent is manganese.

We have discovered that castings of austenitlc white cast iron alloys inwhich the principal alloying agent is manganese may be subjected to adecarburising treatment whereby the average carbon content of suchcastings is decreased and a product is obtained havingphysicalproperties similar to those. associated with austenitic mangan'ese.steel and/or whereby a graded structure with regard to the carboncontent. is developed and a product is obtained which for certain purposes may be equivalent or superior to a similar alloy of homogeneouscarbon content such as austenitic manganese steel.

It is an object of the inventionto provide an improved iron alloy inwhich the'principal alloying ingredient is manganese.

It is another object of the invention to provide an improved iron alloyhaving a special combination of properties. T

It is a further object of the invention to provide an improved ironalloy productshaving. a wear-or abrasion resistant surface and'integraltherewith a tough underlayer. i i

It is also an object of theinvention -,to provide an iron alloy producthaving a surface with the wear and abrasion properties of manganesesteel and having a core with the properties of man'- ganese cast iron. ri

It is still another object of the invention to provide an integral ironalloy product having a hard martensitic' surface supported by a toughaustenitic underlayer land a core of austenitic manganese cast iron.

The invention also contemplates a process of treating castings ofaustenitic manganese white cast iron whereby the aforesaid iron alloysand iron alloy products are produced.

The invention further contemplates a process of treating castings ofaustcnitic manganese white cast iron to impart to the surface thereofproperties similar to those of Wear or abrasion resisting manganesesteel.

It is also withinthe contemplation of the invention to provide castingsof an iron-manganese alloy having an increasing carbon content from thesurface to the core and having a special combination of properties. VOther objects and advantages of the invention will become apparent tothose skilled in the art from the following description.

In accordance with the present invention, castings of austeniticmanganese white cast irons are decarburised by an agent which isrelatively nonoxidising to the manganiferous austenite, but isdecarburising to the carbon content. The decarburising agent may begaseous, liquid or solid.

In carrying the invention into practice, the decarburising agentpreferably consists of a gas or gaseous mixture maintained atsubstantially atmospheric pressure, that is to say, at a pressure of notless than half an atmosphere and not exceeding two atmospheres. Thegaseous decarburising agent may be passed into the treatment chamber ofa furnace to form an atmosphere which, if desired, may be recirculatedby a fan, The re-circulation may be effected within the furnace itselfby means of a fan or through a system which comprises the furnace and anexternal conduit. The re-circulation system may include means forregenerating the atmosphere by adding a controlled supply of air orother oxygen-containing gas whereby the carbon monoxide and hydrogenformed during the decarburising reaction are re-oxidised to carbon di:oxide and water vapour. The system may also include a vent to preventthe setting up of excess internal pressure. Such an atmosphere shouldcontain one or more of the constituents hydro: gen, water vapour andcarbon dioxide as decarburising agents; if carbon dioxide is presentthere should also be. enough carbon monoxide present to reduce oxidationof the austenite to such a degree that the desired dimensions of-thefinished castings will not be detrimentally affected. Similarly thehydrogen-watervapour ratio should be such that the gas mixture isnon-oxidising to the same extent. The mixture may be diluted with inertconstituents such as nitrogen.

Alternatively, in a solid or liquid the gaseous atmosphere to thetreatment chamber, the said atmosphere may be generated within thefurnace itself. Thus, for example, the castings may be treated incontact with molten salts containing oxidising the decarburising agentma be agents or giving rise to the same under the conditions of thereaction, for example by using molten barium chloride, with or withoutbarium oxides or a mixture of sodium chloride and manganese dioxide.When treating the castings with a solid decarburising agent they may beheated in the present of iron ore with or without the addition of smallamounts ofcarhon.

form; also, instead of feeding The cast alloy may be heated, in thepresence of any of the above decarburising agents, to a temperaturebetween about 700 C. and the temperature of incipient fusion of thealloy under treatment, the temperature and duration of treatmentdepending upon the thickness of the section of the casting and theproperties desired.

When the castings are of relatively thick section, and a longdecarburising treatment isnecessary to produce the required averagecarbon content, it may be that there is: an excessively deep zone of lowcarbon at the surface. In this case the castings may be submittedto arecarburising treatment, in a carburising atmosphere by which the carboncontent? of the surfacezone is increased. If a thin surface zone of low:carbon content is desired, a further short decarburising treatment maybe given after the recarburising step.

The composition of the alloy as cast should preferably be in the rangeof about 9% to 25% man anese, about 2% to 6% of carbomnot more thanabout 2.0% of silicon, and not more than about 0.15% of phosphorus. Partof the manganese may be'replaced'by nickel and/or copper to such anextentthat the-alloy may contain up to a total of aboutf24% clinickelandcopper. The nickel will substitute "an: approximately equal weight ofmanganese-whilethe-copper will substitute approximately half its weightof manganese. It will beun derstood 'that the expression fausteniticmanganese cast irons as used herein includes-those alloys-in which partof the manganese is substituted by nickel and/or-copper'but in which themanganese is not less than "5%. Other alloying elements such aschromium, moiybdenum, titanium, tungsten and vanadium may alsobe presentin quantities not exceeding about 5% of any one elementand'not-exceeding about 12% m an, and it-is notintended to exclude thepresence of these elements in -such amounts.

In a preferredapplication of the'inventiomthe cast alloy is decarburisedbya heatitreatment in an atmosphere obtained-by partiallyburning fuelgases such as coal gas, coke oven gas, producer gas, butane, propane orother hydrocarbons. Thus for example when usingan atmosphere producedfrom coal gas, having-Ian approximate composition comprisingabout 45% to48% of hydrogen, about'1'i% to 19% of methane, about 18% to'22i% ofcarbon monoxide,-about 2%:to 4% of carbon dioxide, about 0.5% to 1% ofoxygen, about 6% to 9% of nitrogen, and about'.1.-5% to 2.5% ofilluminants, the gas is mi-xed with-about 2%. 'to 3 parts ofair byvolume and the mixture urnt in a closed combustion chamber with orwithout a, catalyst andthe resulting products of qqmbustion,aftercooling to remove excess water vapour, have an approximatecomposition comprising about 14.% to-8% of hydrogemabout 1 1% to 6% orcarbonmonoxide,:about:6% to 9% of carbon dioxide, about ,1 to.2 of watervapour, the balance being nitrogen. This gas will vary between theselimits depending .upon .the exact gas, to air ratioand uponthe-temperature ofthe combustion chamber and in. regard to the watervapour content upon. the temperature to which theme is co led,Alternatively, other methods removin water vapour an'be employed;

, Th f llowi g s a or; an atmosphere using charcoal producer gals mixedwith air. Charcoal producer gas has an approximat composition comprisingabout 30% 'to'-32% of carbon monoxide, ,about'1% to 3% of carbondioxide, about '2%' to 4% of hydrogen,

example of th production of carbon monoxide, about 7.5

about 1% of water vapour, the balance being nitrogen. This ga is mixedin the ratio of 4 parts of gas with one part of air and is burnt eitherinan externally-heated chamber or in the furnace chamberitself togive'agas having an approximate composition comprising about 20% of carbondioxide,

about 2% of hydrogen, and about 1% of water -vaD01 the balance beingnitrogen.

The process of decarburisation apparently operates in the followingmanner. The gaseous, liquid or solid decarburising medium is out ofchemical equilibrium with the carbon content of the cast iron with theresult that it absorb car- .bon readily, from the surface of thecastings. If gaseousmedium is'used containing hydrogen,

water vapour orcarbon dioxide, the reactions in- 'volved apparently are:

The carbon in the'above reactions is represented for simplicity as.elemental carbon, but in fact is mainly in the'form of metalliccarbides or the solution of carbon in iron known as austenite. Thesurface of theparts being treated rapidly tends to reach equilibriumwith the gas, and this equilibrium carbon content depends upon the ratioof carburising and decarburising constituents of the gaseous mixture atthe temperature of treatment. It is evident, therefore, that the finalsurface carbon contentv can be largely controlled by the gascomposition; Preferably the gas composition is such that it is inequilibrium with a carbon content of between zero and 0.8% so as to getrapid carbon removal. It should be stated that the equilibria alter withthe temperature in accordance with well-known chemical laws and thenecessary 'gas composition may, therefore, vary with the temperature oftreatment. Furthermore, the rateof diffusion of carbon to the surface isgoverned by'the temperature and this is usually the limiting factor.

During the decarburising treatment, carbon is continuously removed fromthe surface of the casting by reaction with the decarburising agent, andsimultaneously carbon diffuses from the centre, where the carbon contentis high, to the surface, where it is low. As the treatment progresses,the depth of the decarburised zone increases, and if carried far enoughwill reach the centre of the section. Continuation of the treatment willcause a lowering of the carbon content at the centre, and a consequentflattening ofv the carbon content gradient from the centre of thesurface; Theoretically, in the limit, a uniform carbon content would beobtained, of a value in equilibrium with the decarburising medium. Inpractice, the time required to approximate this result would be verylong except in the case. of thin sections. The characteristics of thecarbon gradient are continually varying throughout the treatment, andthese characteristicsfor a given thickness of section are influenced bythe time and temperature of treatment, by the composition of thedecarburising medium, bythe original carbon-content of the a1loy,'an'dby thediifusicn constants of carbon in the austenite.

By suitable control of these factors, a carbon content gradient of anydesired characteristics, within certain limits, may be produced.Preferably, in practice, the decarburised castings have a graded carboncontent increasing from a valuein the range of 0.05% to 0.9% at thesurface, to a value in the range of 0.9% to 6.0% at the core.

The metallurgical structure and the physical properties of an alloycontaining manganese in the contents specified, together with the otherelements, will depend upon the carbon content. It isevident, therefore,that a range of metallurgical structures can be produced from thesurface to the core of the section thus treated. For example, aftertreatment and quenching of a casting from a temperature of approximately1000" to 1100 C., the core may consist of a structure substantiallysimilar to that in the original alloy, namely, carbide set in a matrixof austenite. This core will merge into an intermediate zone ofsubstantially pure austenite, the depth or thickness of this zonedepending upon the content of austenite forming elements such asmanganese, nickel or copper and the-time of treatment. Then, approachingthe surface there will be a zone of martensite plus austenite, and withcertain compositions a zone of pure martensite on the surface.

In putting the process into commercial production, normally it is foundthat there is formed a very thin surface skin of ferrite and/or oxidegenerally of a few thousandths of an inch in thickness. This, forcommercial purposes can be disregarded and is incidental to the processand the product. It should, therefore, be understood that, when we referto surface zones, we mean zones immediately underlying said thin skin offerrite and/or oxide.

The physical characteristics of such a section would be somewhat asfollows: The core would be of moderate strength and would possess lowductility, but would be supported by a tough ductile band of austeniteof considerable thickness. The majority of this band would be ofrelatively stable austenite if quenched from a high temperature of theorder of about 1000 to 1100 C. This austenite would become more unstabletowards the surface where it merges intothe hard martensite, Thismartensitic surface is brittle, but is backed by the said toughaustenitic band. The resulting articleas a whole will have a hardwear-resisting surface with a high rate of work hardening, a tough,strong body or intermediate band and acore which is moderately strong,but not ductile. The relative thickness of these bands or 'zones can beadjusted by the time and temperature of treatment and by control of theatmosphere during the various stages of treatment and they will alsovary with the thickness of section. In thin sections the original corestructure may be completely removed and the tough 'austenite structurecan extend right to the centre.

The complex structure resulting from treatment in accordance with thepresent invention may have a possible application in the manufac' tureof armour plating, whilst for other purposes it would give rise to idealwear and abrasion resisting properties combined with adequate strengthand ductility. The resulting product is moreover cheaply and simplyproduced from Zan alloy cast iron which can be readily melted and castby any of, the well-known methods at present in use for cast andmalleable irons.

The process of the invention may be applied to any type of castings.Such castings may include those made by a centrifugal process which areusually of hollow cylindrical shape; after and elongation figures are ameasure of strength about 6% t0 9% tion,;sucli castings could beslitlongitudinally" and rolled, forged, or otherwise manipulated.

If it is desirable that the surface layer of any part ,or parts of acasting should retain its high: carbon content, such part or parts maybe protected from the action of the decarburising agent by any knownprocess for protecting surfaces,

such as by applying an impermeable coating of enamel or metallicdeposit.

The process of the present invention enables products to be producedhaving a wear resistant surface zone containing at least one of theconstituents austenite and martensite and a tough underlying zone orlayer containing at least one of the constituents austenite and carbide.The products preferably have ultimate tensile strengths lying between 18and 45 tons/sq. in. elongations on two inches between 8 and 35 andsurface hardnesses between 200 and 400 V. P. N. with a high rate of workhardening. The tensile and ductility of the material as a whole, and thecapacity for work hardening and surface hardness are a measure of thewear resistance- I of the material.

Material required to have primarily high wear resistance would haveanalysis and treatment to give the highest obtainable surface hardnessandcapacity for work hardening, whereas for toughness, the analysis andtreatment would aim at the optimum tensile and elongation figures.

Although the present invention has been de-' scribed in conjunction withpreferred embodi ments, it is to be understood that modifications andvariations may be resorted to without de-- parting from the spirit andscope of the inven-" tion, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview'and scope of the inventionand appended claims.

Having described our invention what we claim 1 and desire to secure byLetters Patent is:

1. A process of treating a casting of austeniticcarbidie white cast ironhaving a structure com-- prised of austenite and free massive carbidesand containing over 2% and up to about,6% carbon, about 9% to 25%manganese, and the balance essentially iron which comprisesdecarburizing said casting in a solid state at substantially atmosphericpressure and above about 700 C. with a gaseous decarburizing mediumwhich is substantially non-oxidizing to the manganifer-ous austenite insaid casting but is decarburizing to the carbon therein and whichcontains about 8% to 14% hydrogen, about 1% to 2% water vapor, carbondioxide, about 6% to 11% carbon monoxide and the balance nitrogen,continuing said decarburizing treatment for a, period of time sufficientto reduce thecarbon content throughout the entire thickness of thecasting and to produce a graded carbon content increasing toward thecore and comprised when quenched of a work hardenable surface zonehaving a carbon content of less than 0.9% and a structure containingmartensite on an underlayer comprised of unstable austenite merging intoa deeper tough core of more stable austenite from which all free massivecarbides have been removed, and quenching said casting having saidgraded carbon content from approximately 1000 to 1100" C.

2. A process of treating a casting of austeniticcarbidic white cast ironhaving a structure comprised of austenite and free massive carbides and.

treatment in accordance with the present inven- 5 t ining ov r 2% and upto about 6% carbon,

about1'9 to.-- 25%; mang nese=-andthe e-balance essentially iron whichcomprises =-decarburizing said c'astingin-a solid state at substantiallyatmospheric pressureand above-about 700 0-.- with a gaseous:decarburizing medium which is .substantially, non-oxidizing tothemanganiferous austenite in said castingbut-is decarburizing to thecarbon therein -oontinuin 'said .decarburizing treatment for a periodof--.time-suflicient to reduce the carbon :contentf-throughout theentire thickness of thecasting and to=producein the casting a gradedcarbon contentincreasing tow-ard the core andncomprisedw-hen quenched ofa worle hardenable surface zone havingp a carbon content -of--1essthan-0.9 and-a, structure. conj tainingmartensite on-:an underlayercomprised of unstableaustenite-merging into a deeper tough core :of morestableyaustenite from which all free f massive-J carbides: have I beenremoved,

and quenching said casting #having said. graded, carbonvcontentfromapproximately-1000 to-1100-C. 1 3r A-processof treating .a; casting.of austeniticcarbidic white castironhaving astructure comprised:ofaustenite and freemassive carbides and containing over 2% and up toabout-6%- carbon,

24 rotat -least oneof: the 1 elements selected from the-g-roupnconsisting of nickel and. copper, the

amount ofm-anganese plus nickel .plus' one-half thecopper; being; 9%..to25%of. the composition,

and the balance' essentia-lly ironwhich comprises] decarburizing {saidcastingin a solidrstate tat; substantially atmospheric pressure" andabove about '7-00:'- C. w-ith a gaseous decarburizing medium which issubstantially non -exidizing to; the Iman-, ganiferous'austeniteiii-said casting; but is decarburizing to the carbon therein,continuing, said decarburizing treatment for aperiod-of. time.

sufficient: to reduce the carbon content throughout the entire thicknessof-thecastingand to produce in the casting a gradedcarbon contentincreasingwtoward thecore and. comprisedwhen quenched of a workhardenable surface zone having -a carbon content-of less than 0.9% and astructure containing martensiteon. an underlayercomprised of" unstableaustenite] merging intoa deeper tough core of more-stable austenite'from which all free! massive carbides have been removed,,and quenchingsaid .casting having said graded carbon contentfrom approximately 1000to 1-100 C,

42v A-process of treating a castingof austeniticcarbidic white cast ironhavin'gfa. structure comprised-.ofaustenite and free massive carbidesand containingz over 2% ,and toabout 6%, carbon,-

about 9%. to-'25%.; manganese, and the=balance essentially iron whichcomprises decarburizing. said' casting in-a, solid state atsubstantiallyjatmosphericpressure andabove about 700C. with.

a gaseous decarburizing medium which, issubstantially non-oxidizing tothe manganiferous. austenite in said casting but is decarburizing to:

the carbon therein and which contains about 2%- hydrogen, about 1% watervapor, about carbon dioxide, about 20% carbon monoxideand the balancenitrogen, continuing said decarburizing treatment for a period of timesufficient to reduce the.carbon content throughout the entire thicknessvo1 the casting'andto produce a graded.

carbon content increasing toward the core and comprised when quenched ofa work hardenable surface. zone having a carbon content offless than-0.9and a structure containing martensite;

on an underlayer comprised ofiunstab'le austenite.

merginginto a. deeper toughcore of more stable, austeniteirom which allfree massive carbides, have. been removed, and. quenching. said casting.

having said graded carbon content from approximately 10003 to. 1100.?C..

4 ALFRED GORDON Efwn'ss ROBIEIITE;

PETERIRANCIS HANCOCK.

. REEERENCES CITED:

The following references are of record in the file of this "patent? I IUNITED STATES PATENTS OTHER REFERENCES f"A11oy"Cast Irons, 1938, page16. "Revue de Meta1lurgie,. July 1936;pp. 466-472;

Am. Foundrymen.s Assoc.,

